Bump actions/download-artifact from 5 to 6 (#2634)

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 5 to 6.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](actions/download-artifact@v5...v6)

---
updated-dependencies:
- dependency-name: actions/download-artifact
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Hendrik Ranocha <[email protected]>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -188,7 +188,7 @@ jobs:
       # At first, we check out the repository and download all artifacts
       # (and list files for debugging).
       - uses: actions/checkout@v5
-      - uses: actions/download-artifact@v5
+      - uses: actions/download-artifact@v6
       - run: ls -R
       # Next, we merge the individual coverage files and upload
       # the combined results to Coveralls.